The present invention relates to a method of fabricating an optical waveguide device such as an optical filter, and particularly to a method of adjusting the characteristics of a Bragg grating formed in an optical waveguide such as an optical fiber.
In U.S. Pat. No. 5,367,588 published on Nov. 22, 1994, a method of fabricating a Bragg grating using a phase mask is disclosed. According to the method disclosed in the document, a phase mask is placed on an optical fiber having a photorefractive effect characteristic (photoinduced refractive index change caused by ultraviolet (UV) light radiation), and UV light is applied through the phase mask to the optical fiber. The UV light incident on the phase mask are diffracted by the phase mask and interfere with one another, causing interference at a pitch of xcex9, which is a half of the phase mask pitch 2xcex9. Accordingly, at the core of the optical fiber, the photorefractive effect causes the refractive index to change at a pitch of xcex9, forming the Bragg grating in the core of the optical fiber as a result. The Bragg grating reflects input light having a Bragg wavelength xcexB,expressed as xcexB=2neffxcex9, where neff is an effective refractive index in the Bragg grating area.
A demultiplexing or multiplexing optical wavelength filter utilizing Bragg grating is required to have a reflectance of the order of 100%. On the other hand, when a Bragg grating is applied in a gain-flattening filter of an optical amplifier, for instance, a filter having reverse reflection spectrum characteristics to the gain characteristics of the optical amplifier is required. If an optical filter utilizing a Bragg grating is used to choose a wavelength of a semiconductor laser or to stabilize the wavelength, such a reflectance that would minimize the loss of output efficiency of the semiconductor laser must be set.
The dependence of photoinduced refractive index change xcex94n on UV light intensity (that is, the total energy E (J cm2) of UV light applied to a unit area) has logarithmic characteristics, as shown in FIG. 13. FIG. 13 shows that the photoinduced refractive index change xcex94n is prominent (the inclination of the curve in FIG. 13 is steep) when the total energy E of UV light is small and is less prominent (the inclination of the curve in FIG. 13 is gentle) when the total energy E of UV light is large. Conventionally, an area in which the total energy E of UV light is small has been used to form a Bragg grating with low reflectance. Because small fluctuations in fabricating conditions (that is, variance of the total energy E of UV light) have a great effect on the photoinduced refractive index change xcex94n (and consequently to the reflectance), it has been difficult to provide an adequately high reflectance precision (fabricating reproducibility) in comparison with that provided in forming a Bragg grating with high reflectance (high total energy E of UV light).
It is an object of the present invention to provide a method of fabricating an optical waveguide device by which reflection characteristics of a Bragg grating can be precisely adjusted.
According to the present invention, a method of fabricating an optical waveguide device comprises the steps of forming a Bragg grating by applying UV light to an optical waveguide to generate photoinduced refractive index changes; and adjusting characteristics of the Bragg grating by applying UV light for trimming to the optical waveguide. The Bragg grating forming step is executed by applying UV light to the optical waveguide through a phase mask, and the adjusting step is executed by applying UV light for trimming to the optical waveguide not through the phase mask.
Since this method includes the adjusting step that is executed by applying UV light for trimming to the optical waveguide not through the phase mask, reflection characteristics of a Bragg grating can be precisely adjusted.
If the distribution of the total energy of UV light applied to a unit area in the adjusting step is even over the entire longitudinal range of the area in which Bragg grating is formed, the reflectance and the center wavelength of the reflection light of the optical waveguide device to be manufactured can be adjusted.
If the distribution of the total energy of UV light applied to a unit area in the adjusting step is a trapezoidal profile gradually descending in the vicinity of both longitudinal edges of the area in which Bragg grating is formed, the Fabry-Perot interference in the manufactured optical waveguide device is suppressed and the side lobe suppression ratio of the Bragg grating can be improved.
If the adjusting step is executed for a part of the area in which Bragg grating is formed, the manufactured optical waveguide device can be provided with such a property that the reflectance abruptly changes at a desired wavelength.
If the distribution of the total energy of UV light applied to a unit area in the adjusting step gradually increases or decreases along the longitudinal direction of the area in which Bragg grating is formed, the manufactured optical waveguide device can be provided with such a property that the reflectance increases or decreases with increase in wavelength.
If the distribution of the total energy of UV light applied to a unit area in the adjusting step continuously varies along the longitudinal direction of the area in which Bragg grating is formed, the manufactured optical waveguide device can be provided with a desired reflection property (or a transmittance property), particularly a useful property for use as a gain-flattening device.